[heft] Migrate heft-webpack5-plugin to use webpack-dev-server@4.0.x

[bartvandenende-wm]

Summary

heft-webpack5-plugin binds to webpack-dev-server 3.11.x. It looks like 3.11.x currently only provides basic compatibility with webpack 5.0, as open issues are only being addressed in version 4.x.x.

This ticket is a request to consider to update the dependency of webpack-dev-server in heft-webpack5-plugin from @3.11.x to @4.0.x which provides official webpack 5 compatibility.

As 4.x.x. includes breaking changes and just hit the RC.0 status an alternative could be to 'loosen' the semver binding of webpack-dev-server to allow major version overrides on a project/rig basis.

Repro steps

An example of a failure case of webpack-dev-server [3.11.x] is to handle es5 targets properly with HMR:

1. use a web-application sample project
2. in the webpack config set target: ['web', 'es5']
3. notice that HMR is not working

Reference:
webpack/webpack-dev-server#2961

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@rushstack/heft version?	^0.34.7
Operating system?	Mac
Would you consider contributing a PR?	Yes
Node.js version (node -v)?	v12.17.0

[bartvandenende-wm]

it looks like @types/webpack-dev-server are not yet available for 4.x so this likely would be blocking the transition, there is some push for back-porting the fixes to 3.x but no clarity on that either per webpack/webpack-dev-server#3271

[iclanton]

The missing typings might not be a huge issue, unless the API changed in nontrivial ways. Do you know if it changed? Is it enough to just bump the dependency version?

[bartvandenende-wm]

@iclanton looks like there where some breaking changes, below a migration guide:
https://github.com/webpack/webpack-dev-server/blob/master/migration-v4.md Though I'm not sure if it impacts the plugin

The stable version just got release 2 days ago, but no type updates yet from DefinitelyTyped

[bartvandenende-wm]

this issue is solved